1. Technical Field
This disclosure relates generally to a spark plug for internal combustion engines.
2. Background Art
Spark plugs are usually used to ignite fuel in internal combustion engines, such as automobile engines. Japanese Patent First Publication No. 2013-98042 discloses a spark plug equipped with a ground electrode and a center electrode. The ground electrode includes a ground electrode body and a ground electrode chip extending from the ground electrode body toward the center electrode. The ground electrode chip protrudes from the center of a width of the ground electrode body. The spark plug, as taught in the above publication, forms a spark gap between a surface of the ground electrode chip which faces the center electrode and the front end of the center electrode.
The end surface of the ground electrode chip which faces the center electrode slants downward toward the head of the length of the spark plug in a direction in which an air-fuel mixture flows through the spark gap. The spark gap, therefore, has the shortest interval between an upstream edge of the end surface of the center electrode chip and the front end of the center electrode and the longest interval between a downstream edge of the end of the center electrode chip and the front end of the center electrode. In other words, the spark gap gradually increases in the direction of the flow of the air-fuel mixture.
With the above arrangements of the spark plug, an initial spark will be created in the shortest interval of the spark gap which is located on the upstream side of the spark gap. This results in an increase in time it takes for the spark to be carried downward and then blown off in order to ensure the stability in igniting the air-fuel mixture using flame.
In the above spark plug, a starting point on the ground electrode where a spark is developed moves in the downstream direction on the end surface of the ground electrode chip which faces the center electrode. This causes a linear distance between the starting points on the center electrode and the ground electrode to increase and the spark to greatly expand in the downstream direction. Such an increase in linear distance between the starting points of the spark minimizes a risk that ends of the expanded spark are shorted and also facilitates the expansion of the spark, which leads to an increase in area of contact between the air-fuel mixture and the spark.
The above spark plug is, however, designed to have the starting point of a spark on the ground electrode which is movable in a range limited to the size of the ground electrode chip affixed to a portion of the width of the ground electrode body, thus having a limitation in expanding the spark. There is, therefore, still room for improvement in expanding the spark to enhance the stability in igniting the air-fuel mixture.